Ground-based detection of a vibration–rotation line of HD in Orion

The v=1–0 R(5) line of HD at 2.46 μm has been detected at the position of brightest line emission of shocked H2 in the Orion Molecular Cloud. The flux in this HD line, when compared to that of the previously detected HD 0–0 R(5) line at 19.43 μm, suggests that like the v=1 levels of H2, the v=1 levels of HD are populated in LTE, despite their much higher rates of spontaneous emission compared to H2. The higher than expected population of vibrationally excited HD may be due to chemical coupling of HD to H2 via the reactive collisions HD+H⇌H2+D in the shocked gas. The deuterium abundance implied by the strengths of these lines relative to those of H2 is (5.1±1.9)×10−6.